1968 Volume 41 Issue 5 Pages 1175-1179
The preferred rotational conformation of the acetyl group of (+)-10-nor-8-oxoneomenthol (I), (−)-10-nor-8-oxomenthol (II), (−)-10-nor-8-oxocarvomenthol (III), and (+)-10-nor-8-oxoneocarvomenthol (IV) has been examined by a combination of conformational analysis by a priori calculation and variable-temperature circular dichroism and infrared spectrum measurements. Compound I exhibited an inversion in the sign of the Cotton effect upon a change in the solvent. This phenomenon was interpreted as indicating the conformational change Ia\ ightleftarrowsIb. The cause of the change was ascribed to the formation and subsequent rupture of the intramolecular hydrogen bond. Compound II exhibited a solvent-depending circular dichroism curve similar to the curve of I; this was explained as indicating the existence of a conformational equilibrium between the free conformer, IIa, and the internally-hydrogen-bonded conformers, IIb, IIc, and IId. Furthermore, compounds III and IV were found to take preferentially the IIIa–IIIb and IVa–IVb conformations respectively.
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